Among other things, aeration processes are commonly employed to accelerate the bacteriological decomposition of fluid wastes and to prevent algae formation on the surfaces of stagnant ponds and pools. As used herein and in the claims, the term "aeration" can involve the introduction of any type of gas into any type of fluid. Most "aeration" processes involve the use of air.
U.S. Pat. No. 3,969,446 discloses an aerator comprising: an elongate tube having an open upper end and an open lower end; a plurality of turbines mounted in the tube for free rotation; an air header positioned in the bottom of the tube; and flow restriction collars mounted in the tube beneath at least some of the turbines. When the device is vertically submerged in a pond, the upward flow of air through the tube causes the pond fluid to circulate into the bottom and out of the top of the tube. The upward flow of air and fluid through the tube also causes the turbines to rotate. The turbines generate turbulence in the tube and thereby operate to reduce the size, increase the number, and improve the dispersion of the gas bubbles flowing through the tube. The flow restriction collars also increase the amount of turbulence generated within the tube. To further improve aeration efficiency, the turbines employed in the device are preferably configured to rotate in different directions and/or at different speeds.
U.S. Pat. No. 4,374,030 discloses an aeration device which is particularly well-suited for the treatment of waste fluids containing hair and/or other fibrous solid materials and for separating a liquid or solid disperse phase from a continuous liquid phase. The device comprises: a vertical, elongate inner tube have an open upper end; a plurality of turbines mounted for free rotation within the inner tube; a vertical outer tube surrounding the inner tube and having an open upper end positioned above the upper end of the inner tube; a pair of horizontally extending fluid openings provided through the wall of the outer tube near the lower end thereof; a plurality of flow restriction venturis mounted in the inner tube at positions immediately upstream of the turbines; a base member to which the lower ends of the inner and outer tubes are attached and which seals the lower ends of the inner and outer tubes; an air pipe for introducing air into the lower end of the inner tube; and a liquid line extending into the lower interior portion of the inner tube for removing liquid therefrom. The upward flow of air through the inner tube and then through the upper portion of the outer tube causes the turbines to rotate and carries fluid out of the top of the outer tube. The flow of air and fluid out of the top of the outer tube, in turn, operates to draw additional fluid into the device through the openings provided adjacent the bottom of the outer tube. As the fluid flows upward between the outer and inner tubes, the air bubbles exiting the upper end of the inner tube contact the fluid and push the disperse particles or droplets contained in the fluid toward the interior wall of the outer tube. As a result, the fluid separates into two streams. The first stream contains the disperse particles/droplets and is carried by the air stream upward and out of the top of the outer tube. The second fluid stream contains substantially none of the disperse phase and flows downward through the top of the inner tube. A portion of the second stream is eventually removed from the bottom of the inner tube by means of the liquid conduit.
The entire disclosure of U.S. Pat. No. 3,969,446 and the entire disclosure of U.S. Pat. No. 4,374,030 are incorporated herein by reference.
Wastewater lagoons and other treatment ponds tend to stratify into a plurality of layers. These typically include: (a) a top fluid layer having a sufficiently high dissolved oxygen content to support the aerobic digestion of contaminants contained in the fluid; (b) a middle fluid layer having a low dissolved oxygen content (i.e., typically one part per million or less) capable of supporting only facultative bacteria; and (c) a bottom fluid layer wherein only anaerobic digestion occurs. The bottom fluid layer typically contains a large amount of sludge or silt.
In some cases, it is desirable to completely destratify and aerate the pond in order to promote aerobic digestion throughout the waste material. As will be apparent to those skilled in the art, devices of the type disclosed in U.S. Pat. Nos. 3,969,446 and 4,374,030 are highly effective and advantageous for completely destratifying and aerating a pond.
However, in other cases(e.g., in waste water ponds associated with swine farms), the anaerobic digestion processes occurring near the bottom of the pond are relatively complete and efficient. Thus, the bottom layer of the pond can be more efficiently digested by these anaerobic, processes than by mixing and aerating the bottom layer with the upper and middle layers of the pond.
Thus, a need presently exists for an apparatus and method which will effectively and efficiently aerate the upper layer(s) of a pond but will leave the bottom layer of the pond substantially undisturbed (i.e., will not substantially aerate the bottom layer of the pond and/or substantially interfere with the anaerobic activity occurring in the bottom layer).